The many problems associated with adhering dentures to gum tissue by means of adhesives has resulted in the development of various retention means based on the use of magnets. Examples of dental appliances of the general type to which this invention relates can be found in U.S. Pat. Nos. 4,184,252; 4,209,905; 4,302,189; and our copending patent application Ser. No. 06/671,749, filed 11/15/84. Each of these discloses a relatively new generation magnet, specifically a cobalt-rare earth magnet, mounted in an artificial denture in such a way that a full face of the magnet is exposed substantially at the gum tissue-conforming surface of the denture. A magnetizable element is mounted to a structural support associated with the denture wearers jawbone in such a way that a surface thereof is exposed for abutting engagement with the pole face of the magnet in the denture. In one embodiment of the prior art, the magnet is generally cylindrical in shape and magnetically polarized in such a way that only one pole face thereof engages the exposed surface of the magnetizable element. Other embodiments disclose a generally U-shaped magnetic body comprising a spaced-apart parallel pair of elongated bar magnets joined at their adjacent distal ends remote from the magnetizable element by a ferromagnetic bridge, the magnets being inverted with respect to each other so as to be oppositively magnetically polarized. The device disclosed in our aforesaid copending patent application comprises a denture member having a gum-tissue-conforming surface, and a generally U-shaped magnet member secured to the denture member, and having a spaced-apart pair of pole faces which are aligned in a common plane and exposed adjacent the gum-tissue-conforming surface. The said magnet member includes a magnetic bridge element having a spaced-apart pair of surface portions exhibiting opposite magnetic polarity, and a pair of magnetizable leg elements mounted on opposite sides of the magnetic bridge element in intimate abutting contact with substantially the whole of the magnetically polarized surface portions.
One of the disadvantages of certain prior art devices stems from the fact that because in those devices the magnet face and the keeper face are flat, this poses problems when there is loading on the overdenture at points distant from where the magnet-keeper system is located. When such pressure, or loading, especially in the posterior part of the overdenture occurs, a space or gap is caused to appear between the flat faces of the keeper and magnet, which gap disappears when the pressure or load is terminated and the flat faces come together again, that being accompanied by an unwelcome clicking noise.
Attempts to remedy this and other undesirable factors have been manifested in the development of concave/convex mating faces, rather than flat surfaces, on the magnet face and keeper face, to allow the overdenture to move slightly during chewing while remaining in contact with each other without the faces "gaping". These attempts however, have not been entirely satisfactory. One reason for this is that introduction of a curved surface either into the magnet face or the face of the keeper results in undesirable concentrations of force between the two, whenever lateral force is imposed on either an amputated tooth or an implant post to which the keeper is secured. A curved-surface-to-curved-surface relationship works well only when vertical forces are present. Lateral forces drive the cup shape of one member against the dome shape of the other locking one against the other, and resulting in the transfer of such lateral force to the tooth or implant post.
It is well known in the art that lateral forces imposed on a tooth, or teeth, are responsible for much of the periodontal disease which afflicts mankind. By way of further explanation: the only means by which an orthodontist can reposition teeth require the application of a lateral force or forces to exert pressure on those teeth. During this procedure, the bone which is positioned ahead of the tooth being moved, is destroyed. The tooth then moves into this new void--which has resulted from bone destruction. Thus, if a tooth is constantly being subjected to lateral movemcnt, especially lateral movement in all directions, it is apparent that the bone will be destroyed on all sides, eventually, resulting in the loss of the tooth, root, or implant, which ever is being subjected to these lateral forces.